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Abstract:

A protection circuit equipped magnetic energy recovery switch including a
magnetic energy recovery switch having at least two reverse-conductive
type semiconductor switches and two capacitors employs a protection
circuit and control method for protecting the capacitors against an
overvoltage or short-circuited discharge, and protecting the
reverse-conductive type semiconductor switches and a load against an
overvoltage or overcurrent, and can be used a controller or a current
limiter.

Claims:

1.-9. (canceled)

10. A protection circuit equipped magnetic energy recovery switch
comprising: a magnetic energy recovery switch, interposed between an AC
power supply and a load, for storing magnetic energy of a current, and
recovering the magnetic energy to the load; and protection means for
protecting the magnetic energy recovery switch against an overvoltage or
an overcurrent, wherein the magnetic energy recovery switch includes a
bridge circuit having two series-connected reverse-conductive type
semiconductor switches and two series-connected diodes, two
series-connected capacitors respectively connected in parallel to the two
series-connected diodes, and control means that controls phases of gate
control signals for the reverse-conductive type semiconductor switches in
such a way as to alternately perform ON/OFF control on the
reverse-conductive type semiconductor switches in synchronization with a
frequency of the AC power supply, wherein the protection means includes a
voltage detection unit connected between DC terminals of the bridge
circuit to detect a voltage across the two series-connected capacitors, a
discharge circuit connected between the DC terminals of the bridge
circuit and having a discharge resistor and a discharge switch connected
in series to each other, and a current detection unit, interposed between
the AC power supply and the load, for detecting a current which flows
across the load, and wherein the control means controls a gate of the
discharge switch so as to short-circuit the discharge switch when an
output of the voltage detection unit exceeds a predetermined voltage
value, thereby discharging electric charges of the capacitors through the
discharge resistor, and performs current limiting control by making a
duty ratio of an ON pulse of the gate control signal of the
reverse-conductive type semiconductor switch smaller than 0.5 when the
output of the current detection unit exceeds a predetermined current
value.

11. A protection circuit equipped magnetic energy recovery switch
comprising: a magnetic energy recovery switch, interposed between an AC
power supply and a load, for storing magnetic energy of a current, and
recovering the magnetic energy to the load; and protection means for
protecting the magnetic energy recovery switch against an overvoltage or
an overcurrent, wherein the magnetic energy recovery switch includes a
bridge circuit having two series-connected reverse-conductive type
semiconductor switches and two series-connected diodes, first and second
capacitors connected in series to each other and respectively connected
in parallel to the two series-connected diodes, and control means that
controls phases of gate control signals for the reverse-conductive type
semiconductor switches in such a way as to alternately perform ON/OFF
control on the reverse-conductive type semiconductor switches in
synchronization with a frequency of the AC power supply, wherein the
protection means includes, between DC terminals of the bridge circuit, a
first voltage detection unit connected in parallel to the first capacitor
to detect a voltage of the first capacitor, a second voltage detection
unit connected in parallel to the second capacitor to detect a voltage of
the second capacitor, a first discharge circuit connected in parallel to
the first capacitor and having a first discharge resistor and a first
discharge switch connected in series to each other, a second discharge
circuit connected in parallel to the second capacitor and having a second
discharge resistor and a second discharge switch connected in series to
each other, and a current detection unit, interposed between the AC power
supply and the load, for detecting a current which flows across the load,
and wherein the control means controls gates of the discharge switches so
as to short-circuit the first discharge switch when an output of the
first voltage detection unit exceeds a first predetermined value, thereby
discharging electric charge of the first capacitor through the first
discharge resistor, and short-circuit the second discharge switch when an
output of the second voltage detection unit exceeds a second
predetermined value, thereby discharging electric charge of the second
capacitor through the second discharge resistor, and performs current
limiting control by making a duty ratio of an ON pulse of the gate
control signal of the reverse-conductive type semiconductor switch
smaller than 0.5 when the output of the current detection unit exceeds a
predetermined current value.

12. A protection circuit equipped magnetic energy recovery switch
comprising: a magnetic energy recovery switch, interposed between an AC
power supply and a load, for storing magnetic energy of a current, and
recovering the magnetic energy to the load; and protection means for
protecting the magnetic energy recovery switch against an overvoltage or
an overcurrent, wherein the magnetic energy recovery switch includes two
inversely series-connected reverse-conductive type semiconductor
switches, series-connected first and second capacitors connected in
parallel thereto, a wiring connecting an intermediate node between the
two inversely series-connected reverse-conductive type semiconductor
switches and an intermediate node between the two series-connected first
and second capacitors, and control means that controls phases of gate
control signals for the reverse-conductive type semiconductor switches in
such a way as to alternately perform ON/OFF control on the
reverse-conductive type semiconductor switches in synchronization with a
frequency of the AC power supply, wherein the protection means includes a
first voltage detection unit connected in parallel to the first capacitor
to detect a voltage of the first capacitor, a second voltage detection
unit connected in parallel to the second capacitor to detect a voltage of
the second capacitor, a first discharge circuit connected in parallel to
the first capacitor and having a first discharge resistor and a first
discharge switch connected in series to each other, a second discharge
circuit connected in parallel to the second capacitor and having a second
discharge resistor and a second discharge switch connected in series to
each other, and a current detection unit, interposed between the AC power
supply and the load, for detecting a current which flows across the load,
and wherein the control means controls gates of the discharge switches so
as to short-circuit the first discharge switch when an output of the
first voltage detection unit exceeds a predetermined value, thereby
discharging electric charge of the first capacitor through the first
discharge resistor, and short-circuit the second discharge switch when an
output of the second voltage detection unit exceeds a predetermined
value, thereby discharging electric charge of the second capacitor
through the second discharge resistor, and performs current limiting
control by making a duty ratio of an ON pulse of the gate control signal
of the reverse-conductive type semiconductor switch smaller than 0.5 when
the output of the current detection unit exceeds a predetermined current
value.

13. The protection circuit equipped magnetic energy recovery switch
according to claim 1, wherein when the output of the current detection
unit returns to the predetermined value, the control means sets back the
duty ratio of the ON/OFF pulse of the gate control signal of the
reverse-conductive type semiconductor switch to 0.5 and terminates
current limiting control.

14. The protection circuit equipped magnetic energy recovery switch
according to claim 2, wherein when the output of the current detection
unit returns to the predetermined value, the control means sets back the
duty ratio of the ON/OFF pulse of the gate control signal of the
reverse-conductive type semiconductor switch to 0.5 and terminates
current limiting control.

15. The protection circuit equipped magnetic energy recovery switch
according to claim 3, wherein when the output of the current detection
unit returns to the predetermined value, the control means sets back the
duty ratio of the ON/OFF pulse of the gate control signal of the
reverse-conductive type semiconductor switch to 0.5 and terminates
current limiting control.

16. The protection circuit equipped magnetic energy recovery switch
according to any one of claims 1, wherein when a period of time during
which the output of the voltage detection unit is greater than the
predetermined voltage value exceeds a predetermined time, the control
means controls gates of the reverse-conductive type semiconductor
switches so as to turn off all the reverse-conductive type semiconductor
switches to cut off the current.

17. The protection circuit equipped magnetic energy recovery switch
according to any one of claims 2, wherein when a period of time during
which the output of the voltage detection unit is greater than the
predetermined voltage value exceeds a predetermined time, the control
means controls gates of the reverse-conductive type semiconductor
switches so as to turn off all the reverse-conductive type semiconductor
switches to cut off the current.

18. The protection circuit equipped magnetic energy recovery switch
according to any one of claims 3, wherein when a period of time during
which the output of the voltage detection unit is greater than the
predetermined voltage value exceeds a predetermined time, the control
means controls gates of the reverse-conductive type semiconductor
switches so as to turn off all the reverse-conductive type semiconductor
switches to cut off the current.

19. The protection circuit equipped magnetic energy recovery switch
according to any one of claims 1, wherein when a period of time during
which the output of the voltage detection unit is greater than the
predetermined voltage value exceeds a predetermined time, the control
means controls the gate of the discharge switch so as to discharge
electric charges of the two capacitors to set the voltage to
substantially zero, and then controls gates of the reverse-conductive
type semiconductor switches so as to turn on all the reverse-conductive
type semiconductor switches so that the current becomes conductive in
both directions.

20. The protection circuit equipped magnetic energy recovery switch
according to any one of claims 2, wherein when a period of time during
which the output of the voltage detection unit is greater than the
predetermined voltage value exceeds a predetermined time, the control
means controls the gate of the discharge switch so as to discharge
electric charges of the two capacitors to set the voltage to
substantially zero, and then controls gates of the reverse-conductive
type semiconductor switches so as to turn on all the reverse-conductive
type semiconductor switches so that the current becomes conductive in
both directions.

21. The protection circuit equipped magnetic energy recovery switch
according to any one of claims 3, wherein when a period of time during
which the output of the voltage detection unit is greater than the
predetermined voltage value exceeds a predetermined time, the control
means controls the gate of the discharge switch so as to discharge
electric charges of the two capacitors to set the voltage to
substantially zero, and then controls gates of the reverse-conductive
type semiconductor switches so as to turn on all the reverse-conductive
type semiconductor switches so that the current becomes conductive in
both directions.

22. The protection circuit equipped magnetic energy recovery switch
according to any one of claims 1, wherein the control means controls
gates of the reverse-conductive type semiconductor switches so as to turn
off all the reverse-conductive type semiconductor switches to cut off the
current, when an output of the current detection unit exceeds a
predetermined current value.

23. The protection circuit equipped magnetic energy recovery switch
according to any one of claims 2, wherein the control means controls
gates of the reverse-conductive type semiconductor switches so as to turn
off all the reverse-conductive type semiconductor switches to cut off the
current, when an output of the current detection unit exceeds a
predetermined current value.

24. The protection circuit equipped magnetic energy recovery switch
according to any one of claims 3, wherein the control means controls
gates of the reverse-conductive type semiconductor switches so as to turn
off all the reverse-conductive type semiconductor switches to cut off the
current, when an output of the current detection unit exceeds a
predetermined current value.

25. The protection circuit equipped magnetic energy recovery switch
according to claim 1, wherein the control means controls gates of the
reverse-conductive type semiconductor switches so as to turn off only
that of the reverse-conductive type semiconductor switches which is on,
thereby cutting off the current, when an output of the current detection
unit exceeds a predetermined current value.

26. A protection circuit equipped magnetic energy recovery switch
comprising: a magnetic energy recovery switch, interposed between an AC
power supply and a load, for storing magnetic energy of a current, and
recovering the magnetic energy to the load; and protection means for
protecting the magnetic energy recovery switch against an overvoltage or
an overcurrent, wherein the magnetic energy recovery switch includes a
bridge circuit having two series-connected reverse-conductive type
semiconductor switches and two series-connected diodes, two
series-connected capacitors respectively connected in parallel to the two
series-connected diodes, and control means that controls phases of gate
control signals for the reverse-conductive type semiconductor switches in
such a way as to alternately perform ON/OFF control on the
reverse-conductive type semiconductor switches in synchronization with a
frequency of the AC power supply, wherein the protection means includes a
voltage detection unit connected between DC terminals of the bridge
circuit to detect a voltage across the two series-connected capacitors, a
discharge circuit connected between the DC terminals of the bridge
circuit and having a discharge resistor and a discharge switch connected
in series to each other, and wherein when a period of time during which
the output of the voltage detection unit is greater than the
predetermined voltage value exceeds a predetermined time, the control
means controls the gate of the discharge switch so as to discharge
electric charges of the two capacitors to set the voltage to
substantially zero, and then controls gates of the reverse-conductive
type semiconductor switches so as to turn on all the reverse-conductive
type semiconductor switches so that the current becomes conductive in
both directions.

27. A protection circuit equipped magnetic energy recovery switch
comprising: a magnetic energy recovery switch, interposed between an AC
power supply and a load, for storing magnetic energy of a current, and
recovering the magnetic energy to the load; and protection means for
protecting the magnetic energy recovery switch against an overvoltage or
an overcurrent, wherein the magnetic energy recovery switch includes a
bridge circuit having two series-connected reverse-conductive type
semiconductor switches and two series-connected diodes, first and second
capacitors connected in series to each other and respectively connected
in parallel to the two series-connected diodes, and control means that
controls phases of gate control signals for the reverse-conductive type
semiconductor switches in such a way as to alternately perform ON/OFF
control on the reverse-conductive type semiconductor switches in
synchronization with a frequency of the AC power supply, wherein the
protection means includes, between DC terminals of the bridge circuit, a
first voltage detection unit connected in parallel to the first capacitor
to detect a voltage of the first capacitor, a second voltage detection
unit connected in parallel to the second capacitor to detect a voltage of
the second capacitor, a first discharge circuit connected in parallel to
the first capacitor and having a first discharge resistor and a first
discharge switch connected in series to each other, a second discharge
circuit connected in parallel to the second capacitor and having a second
discharge resistor and a second discharge switch connected in series to
each other, and wherein the control means controls gates of the discharge
switches so as to short-circuit the first discharge switch when an output
of the first voltage detection unit exceeds a first predetermined value,
thereby discharging electric charge of the first capacitor through the
first discharge resistor, and short-circuit the second discharge switch
when an output of the second voltage detection unit exceeds a second
predetermined value, thereby discharging electric charge of the second
capacitor through the second discharge resistor, and then controls gates
of the reverse-conductive type semiconductor switches so as to turn on
all the reverse-conductive type semiconductor switches so that the
current becomes conductive in both directions.

28. A protection circuit equipped magnetic energy recovery switch
comprising: a magnetic energy recovery switch, interposed between an AC
power supply and a load, for storing magnetic energy of a current, and
recovering the magnetic energy to the load; and protection means for
protecting the magnetic energy recovery switch against an overvoltage or
an overcurrent, wherein the magnetic energy recovery switch includes two
inversely series-connected reverse-conductive type semiconductor
switches, series-connected first and second capacitors connected in
parallel thereto, a wiring connecting an intermediate node between the
two inversely series-connected reverse-conductive type semiconductor
switches and an intermediate node between the two series-connected first
and second capacitors, and control means that controls phases of gate
control signals for the reverse-conductive type semiconductor switches in
such a way as to alternately perform ON/OFF control on the
reverse-conductive type semiconductor switches in synchronization with a
frequency of the AC power supply, wherein the protection means includes a
first voltage detection unit connected in parallel to the first capacitor
to detect a voltage of the first capacitor, a second voltage detection
unit connected in parallel to the second capacitor to detect a voltage of
the second capacitor, a first discharge circuit connected in parallel to
the first capacitor and having a first discharge resistor and a first
discharge switch connected in series to each other, a second discharge
circuit connected in parallel to the second capacitor and having a second
discharge resistor and a second discharge switch connected in series to
each other, and wherein the control means controls gates of the discharge
switches so as to short-circuit the first discharge switch when an output
of the first voltage detection unit exceeds a predetermined value,
thereby discharging electric charge of the first capacitor through the
first discharge resistor, and short-circuit the second discharge switch
when an output of the second voltage detection unit exceeds a
predetermined value, thereby discharging electric charge of the second
capacitor through the second discharge resistor, and then controls gates
of the reverse-conductive type semiconductor switches so as to turn on
all the reverse-conductive type semiconductor switches so that the
current becomes conductive in both directions.

29. The protection circuit equipped magnetic energy recovery switch
according to claims 26, wherein the protection means further includes a
current detection unit, interposed between the AC power supply and the
load, for detecting a current which flows across the load, and performs
current limiting control by making a duty ratio of an ON pulse of the
gate control signal of the reverse-conductive type semiconductor switch
smaller than 0.5 when the output of the current detection unit exceeds a
predetermined value.

30. The protection circuit equipped magnetic energy recovery switch
according to claims 27, wherein the protection means further includes a
current detection unit, interposed between the AC power supply and the
load, for detecting a current which flows across the load, and performs
current limiting control by making a duty ratio of an ON pulse of the
gate control signal of the reverse-conductive type semiconductor switch
smaller than 0.5 when the output of the current detection unit exceeds a
predetermined value.

31. The protection circuit equipped magnetic energy recovery switch
according to claims 28, wherein the protection means further includes a
current detection unit, interposed between the AC power supply and the
load, for detecting a current which flows across the load, and performs
current limiting control by making a duty ratio of an ON pulse of the
gate control signal of the reverse-conductive type semiconductor switch
smaller than 0.5 when the output of the current detection unit exceeds a
predetermined value.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a protection circuit equipped
magnetic energy recovery switch, and, more particularly, to a protection
circuit equipped magnetic energy recovery switch connected between an AC
power supply and a load and employing a protection circuit and a control
method for protecting capacitors constituting the magnetic energy
recovery switch and recovering and storing magnetic energy against an
overvoltage or short-circuit discharge, and protecting reverse-conductive
type semiconductor switches and a load against an overvoltage or
overcurrent.

BACKGROUND ART

[0002] Nowadays, electric power energy systems are important social
infrastructure which cannot be stopped even for a moment. However, in an
abnormality or a trouble of a load that causes an overcurrent, a measure
taken thereagainst is a high-speed breaking of the load, as is exercised
by a fuse or a high-speed mechanical switch. Nevertheless, there has been
a demand for a high-function switch, so called a controller or a current
limiter, which is capable of limiting only the overcurrent and allowing a
continued operation without the complete stop of the load, as well as a
system recovery to a full operation after the return to its normality.

[0003] Electric power system must be designed to withstand a short-time
overcurrent, such as a rush current at the time an incandescent lamp is
lit, a rush current when an induction motor is started, or an overcurrent
caused by initial excitation inrush of a transformer. It is important to
distribute yield strength of each machine appropriately. A
semiconductor-type inverter power supply in recent years, such as a fuel
battery inverter, for example, cannot withstand, in many cases, a peak
current which is almost ten times the excitation inrush current of a
transformer. Inverter power supplies, therefore, have various soft-start
functions, which work if there is one load for one inverter power supply
but have a difficulty in coping with later-started ones of a plurality of
loads connected to one inverter power supply.

[0004] Electric power systems are designed in consideration of protective
coordination, the current and the duration thereof to withstand an
accidental, short-time overcurrent. However, such systems merely perform
a protective coordination aimed at a prevention of the influence over the
upstream by selectively breaking the accident current by a switch. It is
a recent social demand to achieve a continuous operation as far as
possible without power breaking in an accident that takes place in the
downstream of a system.

[0005] As for a current limiter which limits an accident current with
series elements, an application based on a transient phenomenon between
superconductivity and normal conduction is developed. This is because the
capacity of the breaker becomes extremely large as the accident current
becomes excessively large, so that reduction in accident current to a
half or so, if possible, can reduce the size and cost the breaker. In
addition, such a current limiter is demanded.

[0006] There is a magnetic energy recovery switch (hereinafter called
"MERS") which can perform power control on a load. The MERS includes a
full bridge circuit having four reverse-conductive type semiconductor
switches, and a capacitor connected between the DC terminals of the full
bridge circuit. The capacitor serves to store magnetic energy when the
current is cut off, and recover the magnetic energy to the load. A gate
control signal is sent to the gates of a pair of reverse-conductive type
semiconductor switches positioned diagonally in the full bridge circuit.
The current phase can be controlled by alternately turning on/off the two
pairs of reverse-conductive type semiconductor switches so that one pair
of reverse-conductive type semiconductor switches are turned on when the
other pair of reverse-conductive type semiconductor switches are turned
off by the gate control signal. Further, if the phase of the gate control
signal is controlled in synchronization with the frequency of the AC
power supply, the current phase can be controlled arbitrarily. When the
load connected to the MERS is an inductive load, a voltage to the load
can be increased or decreased by advancing the current phase. This has
already registered as a patent and is disclosed (see Patent Document 1).
The MERS in this mode is called "full-bridge type MERS".

[0007] It has been applied, and laid open, and is publicly known that as
the MERS's include simpler MERS circuits which can be constituted by two
reverse-conductive type semiconductor switches though partly limited the
functions of the full-bridge type MERS are (see Patent Document 2 and
Patent Document 3).

[0008] Of the simple MERS circuits, a so-called vertical half-bridge type
MERS is a half-bridged mode of a full bridge circuit. More specifically,
of the four reverse-conductive type semiconductor switches connected in
bridge in the full-bridge type MERS, two reverse-conductive type
semiconductor switches which are connected to one AC terminal are
replaced with diodes connected in the reverse directions. With a
capacitor having the same capacity being added, the capacitors are
respectively connected in parallel to the diodes.

[0009] Of the simple MERS circuits, a so-called horizontal half-bridge
type MERS is also a half-bridged mode of a full bridge circuit, and
differs from the vertical half-bridge type MERS in a half-bridging
approach. In the horizontal half-bridge circuit a lower half of the full
bridge circuit is used. The lower half means the part that is the lower
one when the full bridge is separated laterally (horizontally) by the
capacitor connected therein. Further, this horizontal half-bridge circuit
has an additional capacitor with the same capacity. More specifically,
two circuits each having a capacitor connected in parallel to the
reverse-conductive type semiconductor switch are connected in series to
each other, and the reverse-conductive type semiconductor switches are
respectively connected in series to each other at the time of connecting
the circuits in series.

[0010] Although the vertical half-bridge type MERS and the horizontal
half-bridge type MERS need twice the quantity of capacitors in use as
compared with the full-bridge type MERS, the quantity of
reverse-conductive type semiconductor switches in user is reduced to a
half. Therefore, the number of reverse-conductive type semiconductor
switches through which the current passes is reduced and conduction loss
is reduced. Because the current duty per a single capacitor (the amount
of the current passing through a capacitor per unit time) is reduced to a
half, the life of capacitors generally becomes longer. In addition, the
basic electric characteristic relating to recovery of magnetic energy
becomes substantially equivalent to that of the full-bridge type MERS.
Both of the vertical half-bridge type MERS and horizontal half-bridge
type MERS are advantageous over the full-bridge type MERS particularly at
the time of application targeting a large current.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

[0011] If the vertical half-bridge type MERS and horizontal half-bridge
type MERS can be used as a controller or a current limiter, the original
intention is achieved. However, problems arise in their usages.

[0012] When the resonance frequency in the vertical half-bridge type MERS
that is determined by the capacitor and the reactance component of a load
is lower than the switching frequency of the gate control signal to be
supplied to the gates of the reverse-conductive type semiconductor
switches, the electric charges of the two capacitors are not discharged
completely and remain, so that a voltage remains in each capacitor
(hereinafter called "offset voltage"; a state where a voltage remains in
a capacitor is called "DC offset mode"). While one capacitor is charging
or discharging, the offset voltage remains in the other capacitor.
Therefore, the voltage in the capacitor which is charging/discharging and
the offset voltage held in the other capacitor are applied to the
reverse-conductive type semiconductor switch in an OFF state, the applied
voltage may exceed the rated voltage of the reverse-conductive type
semiconductor switch and the capacitor.

[0013] When the resonance frequency in the horizontal half-bridge type
MERS that is determined by the capacitor and the reactance component of a
load is lower than the switching frequency of the gate control signal to
be supplied to the gates of the reverse-conductive type semiconductor
switches, an offset voltage is likewise generated in each capacitor; with
the offset voltage being generated in one capacitor, when the other
capacitor is charged, the capacitor where the offset voltage is generated
is subjected to short-circuit discharge (hereinafter the state in the
horizontal half-bridge type MERS where short-circuit discharge of the
capacitor is caused is called "capacitor short-circuiting mode"). In case
of the capacitor short-circuiting mode, short-circuit discharge of the
capacitor is caused, so that an overcurrent flows to the
reverse-conductive type semiconductor switches and the load. This may
damage the reverse-conductive type semiconductor switches.

[0014] It is inevitable from the viewpoint of application that the
vertical half-bridge type MERS and horizontal half-bridge type MERS
should be used while always grasping the electric charge of the capacitor
which affects the withstand voltage of the capacitor and the current
capacity and withstand voltage of the reverse-conductive type
semiconductor switch, i.e., the state of the voltage of the capacitor.

[0015] The DC offset mode and the capacitor short-circuiting mode can be
brought up by the overcurrent of a load as well as the disturbance of the
frequency of the power supply. In addition, the load's overcurrent occurs
more frequently than the disturbance of the frequency of the power
supply. If excess magnetic energy originating from the load's overcurrent
is controlled in the normal operations of the vertical half-bridge type
MERS and the horizontal half-bridge type MERS, unexpected large magnetic
energy may be generated by recovery. At this time, the load voltage
becomes an overvoltage. This overvoltage may exceed the withstand voltage
of the load, causing failure in the load or damaging it. When the
overcurrent of a load occurs, the load is protected if the capacitors and
reverse-conductive type semiconductor switches can be protected promptly.
The function of protecting the capacitors and reverse-conductive type
semiconductor switches is important and eliminates the need for the
capacitors and reverse-conductive type semiconductor switches themselves
to have excessive overload capacities, which is important in reducing the
size of the MERS and reducing the cost.

[0016] In protecting the capacitors and reverse-conductive type
semiconductor switches, it is easy to add a circuit for cutting off the
MERS circuit itself, or a circuit for bypassing the input terminal and
output terminal of the MERS circuit to go to the state of bypassing the
MERS circuit and simply stop the functions of the MERS. However, this
stops the operation of a load or significantly changes the operational
conditions (e.g., changing the full load operation to an intermediate
load operation), which, at the same time, influences another load
operating in parallel to the MERS. Conventionally, this is inevitable.

[0017] The vertical half-bridge type MERS and the horizontal half-bridge
type MERS have two reverse-conductive type semiconductor switches and two
capacitors, can freely set the ON/OFF timing of the reverse-conductive
type semiconductor switches. If the control method which makes the
adequate use of them is used, therefore, it is possible to execute an
operation of inhibiting or limiting the overcurrent while maintaining the
maximum performance without simply cutting off or bypassing the MERS
circuit itself at the time of the overcurrent of a load, and returning
the operation to the normal operation when the factor of causing the
overcurrent is eliminated. If this operation is possible, the functions
of the vertical half-bridge type MERS and the horizontal half-bridge type
MERS can be enhanced.

[0018] The present invention has been made in consideration of those
circumstances, and it is an object of the invention to provide a
protection circuit equipped magnetic energy recovery switch which
protects a vertical half-bridge type MERS and a horizontal half-bridge
type MERS against an overvoltage originating from the offset voltage of a
capacitor in the vertical half-bridge type MERS, a short-circuit
discharge originating from the offset voltage of a capacitor in the
horizontal half-bridge type MERS, and an overvoltage or overcurrent
originating from an abnormality in a load or damage thereof.

Means for Solving the Problems

[0019] The above object of the invention is achieved by the following
means.

[0021] a magnetic energy recovery switch, interposed between an AC power
supply and a load, for storing magnetic energy of a current when the
current is cut off, and recovering the magnetic energy to the load; and

[0022] protection means for protecting the magnetic energy recovery switch
against an overvoltage or an overcurrent,

[0023] wherein the magnetic energy recovery switch includes a bridge
circuit having two series-connected reverse-conductive type semiconductor
switches and two series-connected diodes, two series-connected capacitors
respectively connected in parallel to the two series-connected diodes,
and control means that controls phases of gate control signals for the
reverse-conductive type semiconductor switches in such a way as to
alternately perform ON/OFF control on the reverse-conductive type
semiconductor switches in synchronization with a frequency of the AC
power supply,

[0024] wherein the protection means includes a voltage detection unit
connected between DC terminals of the bridge circuit to detect a voltage
across the two series-connected capacitors, and a discharge circuit
connected between the DC terminals of the bridge circuit and having a
discharge resistor and a discharge switch connected in series to each
other, and

[0025] controls a gate of the discharge switch so as to short-circuit the
discharge switch when an output of the voltage detection unit exceeds a
predetermined value, thereby discharging electric charges of the
capacitors through the discharge resistor.

[0027] a magnetic energy recovery switch, interposed between an AC power
supply and a load, for storing magnetic energy of a current when the
current is cut off, and recovering the magnetic energy to the load; and

[0028] protection means for protecting the magnetic energy recovery switch
against an overvoltage or an overcurrent,

[0029] wherein the magnetic energy recovery switch includes a bridge
circuit having two series-connected reverse-conductive type semiconductor
switches and two series-connected diodes, first and second capacitors
connected in series to each other and respectively connected in parallel
to the two series-connected diodes, and control means that controls
phases of gate control signals for the reverse-conductive type
semiconductor switches in such a way as to alternately perform ON/OFF
control on the reverse-conductive type semiconductor switches in
synchronization with a frequency of the AC power supply,

[0030] wherein the protection means includes, between DC terminals of the
bridge circuit, a first voltage detection unit connected in parallel to
the first capacitor to detect a voltage of the first capacitor, a second
voltage detection unit connected in parallel to the second capacitor to
detect a voltage of the second capacitor, a first discharge circuit
connected in parallel to the first capacitor and having a first discharge
resistor and a first discharge switch connected in series to each other,
a second discharge circuit connected in parallel to the second capacitor
and having a second discharge resistor and a second discharge switch
connected in series to each other, and

[0031] controls gates of the discharge switches so as to short-circuit the
first discharge switch when an output of the first voltage detection unit
exceeds a predetermined value, thereby discharging electric charge of the
first capacitor through the first discharge resistor, and short-circuit
the second discharge switch when an output of the second voltage
detection unit exceeds a predetermined value, thereby discharging
electric charge of the second capacitor through the second discharge
resistor.

[0033] a magnetic energy recovery switch, interposed between an AC power
supply and a load, for storing magnetic energy of a current when the
current is cut off, and recovering the magnetic energy to the load; and

[0034] protection means for protecting the magnetic energy recovery switch
against an overvoltage or an overcurrent,

[0035] wherein the magnetic energy recovery switch includes two inversely
series-connected reverse-conductive type semiconductor switches,
series-connected first and second capacitors connected in parallel
thereto, a wiring connecting an intermediate node between the two
inversely series-connected reverse-conductive type semiconductor switches
and an intermediate node between the two series-connected first and
second capacitors, and control means that controls phases of gate control
signals for the reverse-conductive type semiconductor switches in such a
way as to alternately perform ON/OFF control on the reverse-conductive
type semiconductor switches in synchronization with a frequency of the AC
power supply,

[0036] wherein the protection means includes a first voltage detection
unit connected in parallel to the first capacitor to detect a voltage of
the first capacitor, a second voltage detection unit connected in
parallel to the second capacitor to detect a voltage of the second
capacitor, a first discharge circuit connected in parallel to the first
capacitor and having a first discharge resistor and a first discharge
switch connected in series to each other, a second discharge circuit
connected in parallel to the second capacitor and having a second
discharge resistor and a second discharge switch connected in series to
each other, and

[0037] controls gates of the discharge switches so as to short-circuit the
first discharge switch when an output of the first voltage detection unit
exceeds a predetermined value, thereby discharging electric charge of the
first capacitor through the first discharge resistor, and short-circuit
the second discharge switch when an output of the second voltage
detection unit exceeds a predetermined value, thereby discharging
electric charge of the second capacitor through the second discharge
resistor.

[0038] (4) It is also achieved by the subject matter as specified in any
one of (1) to (3), wherein the protection means further includes a
current detection unit, interposed between the AC power supply and the
load, for detecting a current which flows across the load, and performs
current limiting control by making a duty ratio of an ON pulse of the
gate control signal of the reverse-conductive type semiconductor switch
smaller than 0.5 when the output of the current detection unit exceeds a
predetermined value.

[0039] (5) It is also achieved by the subject matter as specified in (4),
wherein when the output of the current detection unit returns to the
predetermined value, the protection means sets back the duty ratio of the
ON pulse of the gate control signal of the reverse-conductive type
semiconductor switch to 0.5 and terminates current limiting control.

[0040] (6) It is also achieved by the subject matter as specified in any
one of (1) to (4), wherein when a period of time during which the output
of the voltage detection unit is greater than the predetermined value
exceeds a predetermined time, the protection means controls gates of the
reverse-conductive type semiconductor switches so as to turn off all the
reverse-conductive type semiconductor switches to cut off the current.

[0041] (7) It is also achieved by the subject matter as specified in any
one of (1) to (4), wherein when a period of time during which the output
of the voltage detection unit is greater than the predetermined value
exceeds a predetermined time, the protection means controls the gate of
the discharge switch so as to discharge electric charges of the two
capacitors to set the voltage to zero, and then controls gates of the
reverse-conductive type semiconductor switches so as to turn on all the
reverse-conductive type semiconductor switches so that the current
becomes conductive in both directions.

[0042] (8) It is also achieved by the subject matter as specified in any
one of (1) to (3), wherein the protection means further includes a
current detection unit, interposed between the AC power supply and the
load, for detecting a current which flows across the load, and controls
gates of the reverse-conductive type semiconductor switches so as to turn
off all the reverse-conductive type semiconductor switches to cut off the
current, when an output of the current detection unit exceeds a
predetermined value.

[0043] (9) It is also achieved by the subject matter as specified in (1),
wherein the protection means further includes a current detection unit,
interposed between the AC power supply and the load, for detecting a
current which flows across the load, and controls gates of the
reverse-conductive type semiconductor switches so as to turn off only
that of the reverse-conductive type semiconductor switches which is on,
thereby cutting off the current, when an output of the current detection
unit exceeds a predetermined value.

EFFECT OF THE INVENTION

[0044] The invention can provide a protection circuit equipped magnetic
energy recovery switch whose protection circuit can protect a vertical
half-bridge type MERS and a horizontal half-bridge type MERS against an
overvoltage originating from the offset voltage of a capacitor in the
vertical half-bridge type MERS, a capacitor short-circuit discharge
originating from the offset voltage of a capacitor in the horizontal
half-bridge type MERS, and an overvoltage or overcurrent originating from
an abnormality in a load or damage thereof.

[0050]FIG. 5A is a diagram showing the flow of the current of the AC
power supply unit using the conventional horizontal half-bridge type
magnetic energy recovery switch;

[0051]FIG. 5B is a waveform diagram showing the operation when the
current flows as in FIG. 5A and showing the state of a capacitor
short-circuiting mode;

[0052]FIG. 6A is a waveform diagram showing results of simulation of the
generation of an overvoltage and overcurrent in the conventional vertical
half-bridge type magnetic energy recovery switch;

[0053]FIG. 6B is a waveform diagram showing results of simulation of the
generation of an overvoltage and overcurrent in the conventional vertical
half-bridge type magnetic energy recovery switch;

[0054]FIG. 7 is a circuit block diagram showing a first embodiment of a
protection circuit equipped magnetic energy recovery switch according to
the present invention;

[0055]FIG. 8A is a diagram showing results of simulation of the operation
of the first embodiment of the protection circuit equipped magnetic
energy recovery switch according to the invention;

[0056]FIG. 8B is a waveform diagram showing results of simulation of the
operation of the first embodiment of the protection circuit equipped
magnetic energy recovery switch according to the invention;

[0057]FIG. 9A is a diagram showing a circuit for executing another
simulation of the operation of the first embodiment of the protection
circuit equipped magnetic energy recovery switch according to the
invention;

[0058]FIG. 9B is a waveform diagram showing results of another simulation
of the operation of the first embodiment of the protection circuit
equipped magnetic energy recovery switch according to the invention;

[0059]FIG. 9C is a waveform diagram showing results of further simulation
of the operation of the first embodiment of the protection circuit
equipped magnetic energy recovery switch according to the invention;

[0060] FIG. 10 is a diagram illustrating the duty ratio of a gate ON pulse
in the first embodiment of the protection circuit equipped magnetic
energy recovery switch according to the invention;

[0061] FIG. 11 is a circuit block diagram showing a second embodiment of a
protection circuit equipped magnetic energy recovery switch according to
the invention;

[0062] FIG. 12 is a circuit block diagram showing a third embodiment of a
protection circuit equipped magnetic energy recovery switch according to
the invention;

[0063]FIG. 13A is a diagram showing a circuit for executing simulation of
the operation of the third embodiment of the protection circuit equipped
magnetic energy recovery switch according to the invention;

[0064]FIG. 13B is a waveform diagram showing results of simulation of the
operation of the third embodiment of the protection circuit equipped
magnetic energy recovery switch according to the invention;

[0065]FIG. 13C is a waveform diagram showing results of simulation of the
operation of the third embodiment of the protection circuit equipped
magnetic energy recovery switch according to the invention; and

[0066]FIG. 13D is a waveform diagram showing results of simulation of the
operation of the third embodiment of the protection circuit equipped
magnetic energy recovery switch according to the invention.

[0067] 1a, 1b: bridge circuit (half-bridge circuit)

[0068] 3: AC power supply

[0069] 4: control means

[0070] 5, 501, 502: voltage detection unit

[0071] 6, 601, 602: discharge circuit

[0072] 61, 611, 612: discharge resistor

[0073] 62, 621, 622: discharge switch

[0074] 7: current detection unit

[0075] 8: load

[0076] L: reactance component of load

[0077] R: resistive component of load

[0078] 10: protection circuit

[0079] S1, S2, S3, S4: reverse-conductive type semiconductor switch

[0080] G1, G2, G3, G4: gate control signal

[0081] H1, H2, H4: gate control signal

[0082] C1, C2: capacitor

[0083] D1, D2: auxiliary diode

[0084] AC: AC terminal of bridge circuit

[0085] DC(P), DC(N): DC terminal of bridge circuit

[0086] Vin: power supply voltage

[0087] Iin: input current

[0088] Vc1: voltage of capacitor C1

[0089] Vc2: voltage of capacitor C2

[0090] Ic1: current of capacitor C1

[0091] Ic2: current of capacitor C2

[0092] Vout: output voltage

[0093] Iout: output current

BEST MODE FOR CARRYING OUT THE INVENTION

[0094] Preferred embodiments of the present invention will be described
below with reference to the accompanying drawings. To avoid the redundant
description adequately, same reference numerals are given to components,
members and processes which are the same as or equivalent to those shown
in the diagrams. It is to be noted that the embodiments do not limit the
invention but are just illustrative, and all the features of the
embodiments described and combinations thereof should not necessarily be
essential to the invention.

First Embodiment

[0095] Next, a protection circuit equipped magnetic energy recovery switch
according to the first embodiment will be described.

[0096]FIG. 7 is a circuit block diagram showing the configuration of a
protection circuit equipped magnetic energy recovery switch 100 according
to the first embodiment of the invention.

[0098] More specifically, as shown in FIG. 7, the basic configuration of
the protection circuit equipped magnetic energy recovery switch 100 is a
vertical half-bridge type MERS which includes a bridge circuit
(half-bridge circuit) having two reverse-conductive type semiconductor
switches S1, S2 and two auxiliary diodes D1, D2, capacitors C1, C2 which
are connected respectively in parallel to the auxiliary diodes D1, D2
between DC terminals DC(P) and DC(N) of the bridge circuit and store the
magnetic energy of the bridge circuit when the current is cut off, and
control means 4 which controls the phases of gate control signals G1, G2
in such a way as to alternately perform ON/OFF control on the
reverse-conductive type semiconductor switches S1, S2 in synchronization
with the frequency of the power supply.

[0099] The reverse-conductive type semiconductor switch S1, S2 has a
self-turn-off type semiconductor element, such as FET or IGB, and a diode
inversely connected in parallel thereto. When the frequency of the AC
power supply is substantially low, a power MOSFET parasitic diode which
has a long reverse recovery time can be used as the inversely
parallel-connected diode.

[0100] This vertical half-bridge type MERS is interposed between an AC
power supply 3 and a load 8 having a reactance component L and resistive
component R. Specifically, the AC power supply 3 is connected to one AC
terminal (AC) of the bridge circuit, and the load 8 is connected to the
other AC terminal (AC), forming the AC power supply unit 1a.

[0101] A protection circuit 10 for protecting the vertical half-bridge
type MERS against an overvoltage or overcurrent includes a voltage
detection unit 5, which is connected between the DC terminals DC(P) and
DC(N) of the bridge circuit and connected in parallel to the series
circuit of the capacitors C1, C2 and detects the total voltage of the
capacitors C1, C2, and a discharge circuit 6 likewise connected in
parallel to the series circuit of the capacitors C1, C2. The discharge
circuit 6 has a discharge resistor 61 and a discharge switch 62 connected
in series to each other, and the ON/OFF of the discharge switch 62 is
controlled by a gate control signal H1 for the discharge switch 62
supplied from the control means 4.

[0102] More specifically, the output of the voltage detection unit 5 is
input to the control means 4, and is compared with a predetermined value
(threshold value) prestored in the control means 4. When the output of
the voltage detection unit 5 exceeds the threshold value, i.e., when the
total voltage of the capacitor C1 and the capacitor C2 becomes an
overvoltage, the control means 4 sends the gate control signal H1 to
enable the gate of the discharge switch 62 to short-circuit the discharge
switch 62 to discharge electric charges of the capacitor C1 and the
capacitor C2 through the discharge resistor 61, thereby dropping both
capacitor voltages. When the capacitor voltages return to a normal range,
the control means 4 sends the gate control signal H1 to disable the gate
of the discharge switch 62, turning off the discharge switch 62. The
discharge switch 62, like the reverse-conductive type semiconductor
switch S1, S2, may have a self-turn-off type semiconductor element and a
diode inversely connected in parallel thereto, or a power MOSFET.

[0103] Because the voltages of the capacitor C1 and the capacitor C2
oscillate in the periods of gate control signals G1, G2 of the
reverse-conductive type semiconductor switches S1, S2, protection again
the overvoltage should be fast. When the capacitor voltage detected by
the voltage detection unit 5 is going to exceed the threshold value, it
is discharged through the current limiting discharge resistor 61 or the
like, resulting in that the capacitor voltage stays at a value before
exceeding the threshold value.

[0105] When the total voltage of the capacitor C1 and the capacitor C2 is
going to exceed approximately 800 Vpp, the control means 4 turns on the
discharge switch 62 to operate the discharge circuit 6 to discharge the
current to the discharge resistor 61, thereby discharging the
overvoltages of the capacitor C1 and the capacitor C2, so that the total
voltage of the capacitor C1 and the capacitor C2 does not become 800 Vpp
or higher. What is important is that the suppression of the total voltage
of the capacitor C1 and the capacitor C2 results in suppression of the
reactance voltage, so that rises of a load voltage Vout and a load
current lout are also suppressed due to the limitation of the total
voltage of the capacitor C1 and the capacitor C2.

[0106] FIGS. 6A and 6B show simulation results illustrating the operation
of the AC power supply unit 1a using the conventional vertical
half-bridge type MERS which does not have the protection circuit 10.

[0107] After about 0.5 second after an overcurrent is generated when the
resistive component R of the load 8 becomes a half by overload, the
output voltage Vout rapidly rises from 100 Vrms to 240 Vrms and the total
voltage of the capacitor C1 and the capacitor C2 rapidly rises from about
280 pp to about 1600 Vpp. As shown in FIGS. 8A and 8B, therefore, it is
understood that the protection circuit equipped magnetic energy recovery
switch 100 having the protection circuit 10 is effectively working.

[0108] It is the characteristic that although the voltages of the two
capacitors rapidly rise when an overcurrent is generated in the vertical
half-bridge type MERS, both of the overvoltage and overcurrent do not
rise rapidly if the voltages of the capacitors are suppressed. This is
essentially different from overvoltage protection for a capacitor used in
a DC voltage source in the conventional voltage type inverter device.

[0109] As another approach in case of exceeding the withstand voltage of a
capacitor or the withstand voltage of a reverse-conductive type
semiconductor switch, the phase of the gate control signal of the
vertical half-bridge type MERS may be further advanced to reduce shared
voltage of the load. However, the control cycle of the phase of the gate
control signal is a half cycle in synchronization with the frequency of
the AC power supply, so that a change in the ON/OFF phase of the gate
control signal which is close to the phase speed of the AC power supply
makes the ON time of the gate of the reverse-conductive type
semiconductor switch equal to or longer than the half cycle of the period
of the AC power supply and generates a DC component in the load voltage,
which is not favorable. It is necessary to put a time consonant of 10 mS
or greater (when the frequency of the AC power supply is 50 Hz) in a
change in the ON/OFF phase of the gate control signal. The result will
appear one cycle after the period of the AC power supply. While the above
control is sufficient in controlling a change in the output of the load,
it does not fast enough for the protection operation because the current
increasing speed of the overcurrent due to an accident or the like is
faster than one cycle of the AC power supply.

[0110]FIG. 9A is a circuit diagram for simulation to see a rapid change
in the phase of the gate control signal and changes in capacitor voltage,
load voltage and load current which are caused by reduction of the pulse
width of the gate ON signal in the protection circuit equipped magnetic
energy recovery switch 100 according to the first embodiment of the
invention.

[0111]FIG. 9B shows results of simulation in case where the overvoltage
of the load voltage and the overcurrent of the load current occur at time
0.50 second due to overload, and the phase of the gate signal is changed
rapidly at time 0.60 second as a protection measure.

[0112] The waveform diagram of FIG. 9B shows the gate control signals,
load (output) voltage, load (output) current, and capacitor voltages in
order from the top. In this case, when the gate control signal lacks a
pulse, the load (output) current is disturbed and DC current is output.
It is therefore apparent that it is not fast enough to change the ON/OFF
phase of the gate control signal in the direction of reducing the
capacitor voltage when the output of the voltage detection unit 5 is
input to the control means 4, and is compared with a predetermined value
(threshold value) prestored in the control means 4, and when the output
of the voltage detection unit 5 exceeds the threshold value, i.e., when
the total voltage of the capacitor C1 and the capacitor C2 becomes an
overvoltage.

[0113] In this respect, as a further preferable embodiment of the
protection circuit of the present invention, a method of reducing the
pulse width of the ON signal of the gate control signal, i.e., making the
"duty ratio" of the gate control signal smaller than 0.5, may be taken.
That is, as shown in FIG. 7, a current detection unit 7 may be interposed
between the AC power supply unit 1a and the load 8 to detect a current
flown across the load, and when the output of the current detection unit
7 exceeds a predetermined value, the control means 4 may perform current
limiting control by making the ON/OFF "duty ratio" of the pulses in the
gate control signals G1, G2 of the reverse-conductive type semiconductor
switches S1, S2 smaller than 0.5.

[0114] FIG. 10 is for explaining the ON/OFF "duty ratio". In the normal
state or when the output of the current detection unit 7 is not over a
predetermined value, the gate control signal G1 of the reverse-conductive
type semiconductor switch S1 is turned ON at time t1, turned OFF at time
t3 and turned ON again at t5, which is repeated thereafter. The gate
control signal G2 of the reverse-conductive type semiconductor switch S2
is turned OFF at time t1, turned ON at time t3 and turned OFF again at
t5, which is opposite to the action of the gate control signal G1 of the
reverse-conductive type semiconductor switch S1. The ON/OFF switching
timing of the gate control signal leads the timing for switching the
phase of the polarity of the AC power supply by α degrees (i.e.,
the state of the ON/OFF phase α). With regard to the gate control
signal G1 of the reverse-conductive type semiconductor switch S1, the ON
time (from time t1 to time t3) is equal to the OFF time (from time t3 to
time t5). That is, with regard to the gate control signal G2 of the
reverse-conductive type semiconductor switch S2, the OFF time (from time
t1 to time t3) is equal to the ON time (from time t3 to time t5). This is
the state of the ON/OFF "duty ratio"=0.5.

[0115] When the ON/OFF "duty ratio" is made smaller than 0.5, as shown in
FIG. 10, the gate control signal G1 of the reverse-conductive type
semiconductor switch S1 is turned ON at time t1, turned OFF at time t2
(predetermined time before time t3) and turned ON again at t5, which is
repeated thereafter. The gate control signal G2 of the reverse-conductive
type semiconductor switch S2 is turned ON at time t3 and turned OFF again
at t4 (predetermined time before time t5), which is opposite to the
action of the gate control signal G1 of the reverse-conductive type
semiconductor switch S1. Here, the ON time from time t2 to time t3 is
deleted for the gate control signal G1 of the reverse-conductive type
semiconductor switch S1, and the ON time from time t4 to time t5 is
deleted for the gate control signal G2 of the reverse-conductive type
semiconductor switch S2. That is, the pulse width of the ON signal is
reduced.

[0116] The duration from time t2 to time t3 and the duration from time t4
to time t5 are a duration in which the gate control signal G1 of the
reverse-conductive type semiconductor switch S1 and the gate control
signal G2 of the reverse-conductive type semiconductor switch S2 are both
OFF (hereinafter called "all-OFF duration"). The all-OFF duration is set
at rear portions of the ON durations of the gate control signal G1 of the
reverse-conductive type semiconductor switch S1 and the gate control
signal G2 of the reverse-conductive type semiconductor switch S2 (rear
portion of the ON duration is cut). The all-OFF duration may be set once
in the ON duration, or may be provided by instantaneously disabling the
gate which is enabled multiple times.

[0117]FIG. 9C shows results of simulation in case where the overvoltage
of the load voltage and the overcurrent of the load current occur at time
0.50 second due to overload, and the pulse width of the ON signal is
reduced rapidly from "duty ratio"=0.5 at time 0.60 second as a protection
measure.

[0118] The waveform diagram of FIG. 9C shows the gate control signals,
load (output) voltage, load (output) current, and capacitor voltages in
order from the top. It is apparent that even if the pulse width of the ON
signal of the gate control signal is reduced, the current is reduced
without disturbance in the current waveform.

[0119] Specifically, the control means 4 reduces the pulse width of the ON
signal of the gate control signal G1, G2 of the reverse-conductive type
semiconductor switch S1, S2 with an overvoltage signal upon detection of
an overvoltage or with an overcurrent signal upon detection of an
overcurrent. Accordingly, the rise in load voltage stops, and the load
current is decreased to or below the overcurrent protection level under
the current feedback control. The overvoltages of the capacitor C1 and
the capacitor C2 are likewise lowered below the protection level. Because
the effect of the current feedback control based on the control on the
duty ratio of the gate control signal does not appear instantaneously, it
is desirable to combine the control with discharge of electric charge of
the capacitor by means of the discharge switch 62. In addition, it is
desirable to fix the ON/OFF phase unchanged at the time of executing the
current feedback control based on the duty ratio control on the gate
control signal and/or discharge with the discharge switch 62.

[0120] When the load current exceeds the threshold value, the current
feedback control based on the duty ratio control on the gate control
signal makes the gate-OFF time of the reverse-conductive type
semiconductor switch S1, S2 longer (pulse width of the ON signal
narrower). That is, the reverse-conductive type semiconductor switches
S1, S2 work as a current limiter. Therefore, the vertical half-bridge
type MERS itself can be operated as a current limiter.

[0121] As apparent from the above, the overvoltage protecting function by
cutting the peaks of the voltages of the capacitor C1 and the capacitor
C2 and the current limiting function based on the duty ratio control on
the gate control signals G1, G2 of the reverse-conductive type
semiconductor switches S1, S2 are available as the method of protecting
the vertical half-bridge type MERS. One of the functions may be used or
they may be combined. In case of the combination, the protecting function
for the vertical half-bridge type MERS is further enhanced.

[0122] The configuration may be made to control the gate control signals
G1, G2 in such a way that when the duration in which the output of the
voltage detection unit 5 exceeds a predetermined value exceeds a
predetermined time, the reverse-conductive type semiconductor switches
S1, S2 are both turned off to cut off the current.

[0123] The configuration may be made to control the gate control signals
G1, G2 in such a way that when the duration in which the output of the
voltage detection unit 5 exceeds a predetermined value exceeds a
predetermined time, the gate of the discharge switch is controlled so
that the electric charges of the capacitor C1 and the capacitor C2 are
discharged to have a voltage of zero, after which the reverse-conductive
type semiconductor switches S1, S2 are both turned on to set the current
conductive in both directions.

Second Embodiment

[0124] Next, a protection circuit equipped magnetic energy recovery switch
according to the second embodiment will be described.

[0125] FIG. 11 is a circuit block diagram showing the configuration of a
protection circuit equipped magnetic energy recovery switch 200 according
to the second embodiment of the invention.

[0127] In the second embodiment of the invention, two discharge circuits
and two voltage detection units corresponding to the capacitor C1 and the
capacitor C2 are provided as a protection circuit.

[0128] More specifically, a first discharge circuit 601 is connected in
parallel to the capacitor C1, a second discharge circuit 602 is connected
in parallel to the capacitor C2, a first voltage detection unit 501 is
connected in parallel to the capacitor C1 and the first discharge circuit
601, and a second voltage detection unit 502 is connected in parallel to
the capacitor C2 and the second discharge circuit 602.

[0129] The first discharge circuit 601 has a discharge resistor 611 and a
discharge switch 621 connected in series to each other, and the second
discharge circuit 602 has a discharge resistor 612 and a discharge switch
622 connected in series to each other. The ON/OFF of each discharge
switch 621, 622 is controlled by the gate control signal H1, H2 supplied
from the control means 4. That is, when the output of the voltage
detection unit 501 is input to the control means 4, and is compared with
a predetermined value (threshold value) prestored in the control means 4,
and when the output of the voltage detection unit 501 exceeds the
threshold value, i.e., when the voltage of the capacitor C1 becomes an
overvoltage, the control means 4 sends the gate control signal H1 to
enable the gate of the discharge switch 621 to short-circuit the
discharge switch 621 to discharge the electric charge of the capacitor C1
through the discharge resistor 611, thereby dropping the capacitor
voltage. When the capacitor voltage returns to the normal range, the
control means 4 sends the gate control signal H1 to disable the gate of
the discharge switch 621, turning off the discharge switch 621.

[0130] Likewise, when the output of the voltage detection unit 502 is
input to the control means 4, and is compared with the predetermined
value (threshold value) prestored in the control means 4, and when the
output of the voltage detection unit 502 exceeds the threshold value,
i.e., when the voltage of the capacitor C2 becomes an overvoltage, the
control means 4 sends the gate control signal H2 to enable the gate of
the discharge switch 622 to short-circuit the discharge switch 622 to
discharge the electric charge of the capacitor C2 through the discharge
resistor 612, thereby dropping the capacitor C2 voltage. When the
capacitor C2 voltage returns to the normal range, the control means 4
sends the gate control signal H2 to disable the discharge switch 622,
turning off the discharge switch 622.

[0131] According to the second embodiment of the invention, as apparent
from the above, it is possible to individually detect the voltages of the
capacitor C1 and the capacitor C2 of the vertical half-bridge type MERS
by means of the voltage detection unit 501 and the voltage detection unit
502, to thereby individually protect the capacitor C1 and the capacitor
C2.

[0132] Further, as shown in FIG. 11, the current detection unit 7 may be
interposed between the AC power supply unit 1a and the load 8 to detect
the current flowing across the load 8, and current limiting control may
be carried out in such a way that when the output of the current
detection unit 7 exceeds a predetermined value, the control means 4 makes
the ON/OFF "duty ratio" of the pulses of the gate control signals G1, G2
of the reverse-conductive type semiconductor switches S1, S2 smaller than
0.5. The mode of the "duty ratio" is similar to the one described in the
description of the protection circuit equipped magnetic energy recovery
switch 100 according to the first embodiment of the invention.

[0133] The configuration may be made to control the gate control signals
G1, G2 in such a way that when the duration in which the output of the
voltage detection unit 501, 502 exceeds a predetermined value exceeds a
predetermined time, the reverse-conductive type semiconductor switches
S1, S2 are both turned off to cut off the current.

[0134] In addition, the configuration may be made to control the gate
control signals G1, G2 in such a way that when the duration in which the
output of the voltage detection unit 501, 502 exceeds a predetermined
value exceeds a predetermined time, the gate H1, H2 of the discharge
switch 621, 622 is controlled so that the electric charges of the
capacitor C1 and the capacitor C2 are discharged to have a voltage of
zero, after which the reverse-conductive type semiconductor switches S1,
S2 are both turned on to set the current conductive in both directions.

Third Embodiment

[0135] Next, a protection circuit equipped magnetic energy recovery switch
according to the third embodiment will be described.

[0136] FIG. 12 is a circuit block diagram showing the configuration of a
protection circuit equipped magnetic energy recovery switch 300 according
to the third embodiment of the invention.

[0138] More specifically, as shown in FIG. 12, the protection circuit
equipped magnetic energy recovery switch 300 has two inversely
series-connected reverse-conductive type semiconductor switches S2, S4,
series connection of a first capacitor C1 and second capacitor C2
connected in parallel to the reverse-conductive type semiconductor
switches S2, S4, a wiring 23 connecting an intermediate node between the
two reverse-conductive type semiconductor switches S1 and S2 and an
intermediate node between the first capacitor C1 and the second capacitor
C2, and control means 4 which controls the phases of gate control signals
G2, G4 in such a way as to alternately perform ON/OFF control on the
reverse-conductive type semiconductor switches in synchronization with
the frequency of the AC power supply. The AC power supply 3 is connected
to the connection node between the reverse-conductive type semiconductor
switch S2 and the capacitor C1, and the load 8 is connected to the
connection node between the reverse-conductive type semiconductor switch
S4 and the capacitor C2 via the current detection unit 7.

[0139] In the third embodiment of the invention, the first discharge
circuit 601 and C1 are connected in parallel to each other, the second
discharge circuit 602 and the capacitor 22 are connected in parallel to
each other, the first voltage detection unit 501 is connected in parallel
to the capacitor 21 and the first discharge circuit 601, and the second
voltage detection unit 502 is connected in parallel to the capacitor 22
and the second discharge circuit 602.

[0140] The first discharge circuit 601 has the discharge resistor 611 and
the discharge switch 621 connected in series to each other, and the
second discharge circuit 602 has the discharge resistor 612 and the
discharge switch 622 connected in series to each other. The ON/OFF of
each discharge switch 621, 622 is controlled by a gate control signal H2,
H4 supplied from the control means 4.

[0141] That is, when the output of the voltage detection unit 501 is input
to the control means 4, and is compared with a predetermined value
(threshold value) prestored in the control means 4, and when the output
of the voltage detection unit 501 exceeds the threshold value, i.e., when
the voltage of the capacitor C1 becomes an overvoltage, the control means
4 sends the gate control signal H2 to enable the gate of the discharge
switch 621 to short-circuit the discharge switch 621 to discharge the
electric charge of the capacitor C1 through the discharge resistor 611,
thereby dropping the capacitor C1 voltage. When the capacitor C1 voltage
returns to the normal range, the control means 4 sends the gate control
signal H2 to disable the gate of the discharge switch 621, turning off
the discharge switch 621.

[0142] Likewise, when the output of the voltage detection unit 502 is
input to the control means 4, and is compared with the predetermined
value (threshold value) prestored in the control means 4, and when the
output of the voltage detection unit 502 exceeds the threshold value,
i.e., when the voltage of the capacitor C2 becomes an overvoltage, the
control means 4 sends the gate control signal H4 to enable the gate of
the discharge switch 622 to short-circuit the discharge switch 622 to
discharge the electric charge of the capacitor C2 through the discharge
resistor 612, thereby dropping the capacitor C2 voltage. When the
capacitor C2 voltage returns to the normal range, the control means 4
sends the gate control signal H4 to disable the discharge switch 622,
turning off the discharge switch 622.

[0143] According to the third embodiment of the invention, as apparent
from the above, it is possible to individually protect the capacitor C1
and the capacitor C2 by individually detecting the voltages of the
capacitor C1 and the capacitor C2 of the horizontal half-bridge type MERS
by means of the voltage detection unit 501 and the voltage detection unit
502.

[0144] Further, as shown in FIG. 12, the current detection unit 7 may be
interposed between the AC power supply unit 1b and the load 8 to detect
the current flowing across the load 8, and current limiting control may
be carried out in such a way that when the output of the current
detection unit 7 exceeds a predetermined value, the control means 4 makes
the ON/OFF "duty ratio" of the pulses of the gate control signals G2, G4
of the reverse-conductive type semiconductor switches S2, S4 smaller than
0.5. The mode of the "duty ratio" is similar to the one described in the
description of the protection circuit equipped magnetic energy recovery
switch 100 according to the first embodiment of the invention.

[0145]FIG. 13A is a circuit diagram for simulation to see changes in
capacitor voltage, load (output) voltage and load (output) current which
are caused by reduction of the pulse width of the gate ON signal in the
overvoltage protection circuit in the protection circuit equipped
magnetic energy recovery switch 300 according to the third embodiment of
the invention.

[0146]FIG. 13B shows results of simulation of changes in load (output)
voltage and load (output) current in case where the overvoltage is
discharged by the discharge circuit, when the overvoltage of the load
voltage and the overcurrent of the load current occur between time 0.20
second and time 0.50 second due to overload, and at the same time, when
the capacitors C1, C2 show voltages equal to or higher than a
predetermined voltage.

[0147] The waveform diagram of FIG. 13B shows the gate control signals,
load (output) voltage, capacitor voltages, load (output) current and the
gate control signals in the discharge circuits in order from the top. It
is apparent that in case where the capacitor C1 and the capacitor C2 show
voltages equal to or higher than a predetermined voltage, the discharge
switches 621, 622 discharge the overvoltages of the capacitor C1 and the
capacitor C2, so that the voltages of the capacitor C1 and the capacitor
C2 are suppressed to approximately 400 Vpp, resulting in suppression of
significant increases in load voltage Vout and load current lout.

[0148]FIG. 13C shows results of simulation of changes in load (output)
voltage and load (output) current in case of taking a measure of not
disabling the gate G4 of the gates G2, S4 of the reverse-conductive type
semiconductor switch S2 when the overvoltage of the load voltage and the
overcurrent of the load current occur between time 0.20 second and time
0.50 second due to overload, and at the same time, when the voltages of
the capacitors C1, C2 are not zero.

[0149] The waveform diagram of FIG. 13C shows the gate control signals,
load (output) voltage, capacitor voltages, and load (output) current in
order from the top. At the time of switching ON/OFF the
reverse-conductive type semiconductor switches S2, S4, if the voltage of
the capacitor connected in parallel to the reverse-conductive type
semiconductor switch is not zero, the control means 4 keeps the previous
state (state where the reverse-conductive type semiconductor switch is
OFF) until the voltage of the capacitor becomes zero, and supplies the
gate control signal to turn on the reverse-conductive type semiconductor
switch on when the voltage of the capacitor becomes zero. Therefore,
short-circuit discharge does not occur in the capacitor C1 and the
capacitor C2.

[0150] The pulse waveforms of the gate control signals G2, G4 do not
enable the gates G2, G4 when the voltages of the capacitors C1, C2 are
not zero even in the duration where the gates G2, G4 of the
reverse-conductive type semiconductor switches are ON. This is equivalent
to reducing the pulse widths of the pulse ON signals of the gate control
signals G2, G4 of the reverse-conductive type semiconductor switches S2,
S4, so that the "duty ratio" is smaller than 0.5.

[0151] Therefore, even in the protection circuit equipped magnetic energy
recovery switch 300 according to the third embodiment of the invention,
to protect the capacitors C1, C2, it is effective to reduce the pulse
widths of the pulse ON signals of the gate control signals G2, G4 of the
reverse-conductive type semiconductor switches S2, S4, thereby making the
"duty ratio" smaller than 0.5. The all-OFF duration is set at front
portions of the ON durations of the gate control signal G1 of the
reverse-conductive type semiconductor switch S1 and the gate control
signal G2 of the reverse-conductive type semiconductor switch S2 (first
portion of the ON duration is cut).

[0152] Although the all-OFF duration is set by restriction of inhibiting
the gate control signals G2, G4 of the reverse-conductive type
semiconductor switches S2, S4 from being enabled when the voltages of the
capacitor C1 and the capacitor C2 are not zero, the all-OFF duration may
be set in consideration of the times of discharging the capacitor C1 and
the capacitor C2.

[0153] As described above, the overvoltage protecting function by cutting
the peaks of the voltages of the capacitor C1 and the capacitor C2 and
the current limiting function based on the duty ratio control on the gate
control signals G2, G4 of the reverse-conductive type semiconductor
switches S2, S4 are available as the method of protecting the horizontal
half-bridge type MERS. One of the functions may be used or they may be
combined. In case of the combination, the protecting function for the
horizontal half-bridge type MERS is further enhanced.

[0154]FIG. 13D shows results of simulation in case a measure of not
enabling the gates G2, G4 and a measure of discharging the overvoltage
using the discharge circuit are both carried out when the overvoltage of
the load voltage and the overcurrent of the load current occur between
time 0.20 second and time 0.50 second due to overload, and at the same
time, when the voltages of the capacitor C1 and the capacitor C2 are not
zero.

[0155] The waveform diagram of FIG. 13D shows the gate control signals,
load (output) voltage, capacitor voltages, load (output) current and the
gate control signals in the discharge circuits in order from the top. It
is apparent that in this case too, the voltages of the capacitor C1 and
the capacitor C2 are suppressed to approximately 400 Vpp, and significant
increases in load voltage Vout and load current lout are suppressed.
Further, the load (output) current less than that in the steady state is
supplied, showing a greater effect of current limiting control.

[0156] Even in the protection circuit equipped magnetic energy recovery
switch 300 according to the third embodiment of the invention, it is
desirable, at the time of executing the current feedback control based on
the duty ratio control on the gate pulse and/or discharge with the
discharge switches 621, 622, to fix the ON/OFF phase unchanged.

[0157] The configuration may be made to control the gate control signals
G2, G4 in such a way that when the duration in which the output of the
voltage detection unit 501, 502 exceeds a predetermined value exceeds a
predetermined time, the gate H2, H4 of the discharge switch 621, 622 is
controlled so that the electric charges of the capacitor C1 and the
capacitor C2 are discharged to have a voltage of zero, after which the
reverse-conductive type semiconductor switches S2, S4 are both turned on
to set the current conductive in both directions.

[0158] It is to be noted that the invention is not limited to the
foregoing embodiment, are to be considered as illustrative and not
restrictive and the invention is not to be limited to the details given
herein, and modifications, such as various design changes, may be made
based on the knowledge of those skilled in the art, and embodiments
having such modifications made thereto are included within the scope of
the invention.